Rupture process and aftershock focal mechanisms of the 2022 M6.8 Luding earthquake in Sichuan

According to the China Earthquake Networks Center, a strong earthquake of M6.8 occurred in Luding County, Ganzi Tibetan Autonomous Prefecture, Sichuan Province, China (102.08°E, 29.59°N), on September 5, 2022, with a focal depth of 16 km. Rapid determination of the source parameters of the earthquake sequence is vital for post-earthquake rescue, disaster assessment, and scientific research. Near-field seismic observations play a key role in the fast and reliable determination of earthquake source parameters. The numerous broadband seismic stations and strong-motion stations recently deployed by the National Earthquake Intensity Rapid Report and Early Warning project have provided valuable real-time near-field observation data. Using these near-field observations and conventional mid- and far-field seismic waveform records, we obtained the focal mechanism solutions of the mainshock and M ≥ 3.0 aftershocks through the waveform fitting method. We were further able to rapidly invert the rupture process of the mainshock. Based on the evaluation of the focal mechanism solution of the mainshock and the regional tectonic setting, we speculate that the Xianshuihe fault formed the seismogenic structure of the M6.8 strong earthquake. The aftershocks formed three spatially separated clusters with distinctly different focal mechanisms, reflecting the segmented nature of the Xianshuihe fault. As more high-frequency information has been applied in this study, the absolute location of the fault rupture is better constrained by the near-field strong-motion data. The rupture process of the mainshock correlates well with the spatial distribution of aftershocks, i.e., aftershock activities were relatively weak in the maximum slip area, and strong aftershock activities were distributed in the peripheral regions.     

High-resolution structure-from-motion models covering 160 km-long surface ruptures of the 2021 MW 7.4 Madoi earthquake in northern Qinghai-Tibetan Plateau

The 22 May 2021 M_W 7.4 Madoi, Qinghai, China earthquake presented a rare opportunity to apply the modern unmanned aerial vehicle (UAV) photography method in extreme altitude and weather conditions to image surface ruptures and near-field effects of earthquake-related surface deformations in the remote Tibet. High-resolution aerial photographs were acquired in the days immediately following the mainshock. The complex surface rupture patterns associated with this event were covered comprehensively at 3–6 cm resolution. This effort represents the first time that an earthquake rupture in the interior of the Qinghai-Tibetan Plateau has been fully and systematically captured by such high-resolution imagery, with an unprecedented level of detail, over its entire length. The dataset has proven valuable in documenting subtle and transient rupture features, such as the significant mole-tracks and opening fissures, which were ubiquitous coseismically but degraded during the subsequent summer storm season. Such high-quality imagery also helps to document with high fidelity the fractures of the surface rupture zone (supplements of this paper), the pattern related to how the faults ruptured to the ground surface, and the distribution of off-fault damage. In combination with other ground-based mapping efforts, the data will be analyzed in the following months to better understand the mechanics of earthquake rupture related to the fault zone rheology, rupture dynamics, and frictional properties along with the fault interface.     

Reconstruction of the Changjiang River tidal-limit movement since the ninth century AD

The tide can now propagate upstream for more than 600 km from the Changjiang River estuary to hinterland,which is rare in the world. In China,information and records reflecting tidal limit and its movement in the Changjiang River can be found from some ancient books,literary works and local chronicles. At the end of the 3rd and the beginning of the 4th century AD, the tide limit of the Changjiang River reached the upstream of Chaisang(now named Jiujiang in Jiangxi Province). At the early 9th century,the tide limit in the dry season retreated to the Penpukou in Jiujiang. In the last ten years of the 13th century,the tidal limit moved back to the Xiaogushan Mountain near the border between Anhui Province and Jiangxi Province. By the middle of the 20th century,the modern researchers generally agreed that the tidal limit retreated to Datong in Anhui Province. Before the main stream of the Changjiang River was cut off by large-scale water conservancy projects,the tide limit had retreated seaward continuously. From the 3rd decade of the 9th century to the last decade of the 13th century,the tide limit moved downstream 64 km during the 470 years,with an average annual rate of 0.136km/a. During the 670 years from the end of the 13th century to the middle of the 20th century,the tide limit had moved downstream 170 km,with an average annual rate of 0.254 km/a,almost twice as much as that of the previous period. We suggest the climate change accounted for the different retreat rate of the tidal limit between the two periods discussed. In addition,a recent study found that during the extremely dry periods,when the discharge of Jiujiang station was about 8440 m³/s,the tidal limit reached near Jiujiang. In the years when the water conservancy project on the main stream of the Changjiang River began to store water,the tide limit actually returned to the previous position of the middle Tang Dynasty,i.e. 1100 years ago.     

Geometry,spatial arrangement and origin of carbonate grain‐dominated,scour‐fill and event‐bed deposits: Late Jurassic Jubaila Formation and Arab‐D Member,Saudi Arabia

Outcrop analogues of the Late Jurassic lower Arab‐D reservoir zone in Saudi Arabia expose a succession of fining‐upward cycles deposited on a distal middle‐ramp to outer‐ramp setting. These cycles are interrupted by erosional scours that incise up to 1·8 m into underlying deposits and are infilled with intraclasts up to boulder size (1 m diameter). Scours of similar size and infill are not commonly observed on low‐angle carbonate ramps. Outcrops have been used to characterize and quantify facies‐body geometries and spatial relationships. The coarse grain size of scour‐fills indicates scouring and boulder transport by debris or hyperconcentrated density flows strengthened by offshore‐directed currents. Longitudinal and lateral flow transformation is invoked to produce the ‘pit and wing’ geometry of the scours. Scour pits and wings erode up to 1·8 m and 0·7 m deep, respectively, and are on average 50 m wide between wing tips. The flat bases of the scours and their lack of consistent aspect ratio indicate that erosion depth was limited by the presence of cemented firmgrounds in underlying cycles. Scours define slightly sinuous channels that are consistently oriented north–south, sub‐parallel to the inferred regional depositional strike of the ramp, suggesting that local palaeobathymetry was more complex than commonly assumed. Weak lateral clustering of some scours indicates that they were underfilled and reoccupied by later scour incision and infill. Rudstone scour‐fills required reworking of material from inner ramp by high‐energy, offshore‐directed flows, associated with storm action and the hydraulic gradient produced by coastal storm setup, to generate erosion and sustain transport of clasts that are generally associated with steeper slopes. Quantitative analysis indicates that these coarse‐grained units have limited potential for correlation between wells as laterally continuous, highly permeable reservoir flow units, but their erosional and locally clustered character may increase effective vertical permeability of the Arab‐D reservoir zone as a whole.     

静水条件下冰冻泥沙水下休止角的试验研究

为研究冰冻泥沙在静水中融化后休止角的变化规律以及不同环境中泥沙休止角的差异,采用粒径为0～1 mm、1～2 mm、2～4 mm的泥沙试样,进行了风干泥沙在空气中的休止角、无冰泥沙以及冰冻泥沙水下休止角试验。试验结果表明：对于同一粒径范围的泥沙,风干沙在空气中的休止角最大,冰冻泥沙水下休止角次之,无冰泥沙水下休止角最小,三种试验情景中休止角均随泥沙粒径的增大而增大;泥沙水下休止角与风干沙在空气中休止角的差异,主要与水对泥沙的润滑作用以及泥沙在不同介质中的有效容重系数有关;泥沙在水下受到冰的影响表现为局部泥沙颗粒滑动,这更有利于斜坡上泥沙的堆积,相对于无冰泥沙而言,冰的存在使泥沙水下休止角增大。     

开挖卸荷条件下抗拔桩破坏机理及承载力分析

随着大型基础施工进度和地层限制等要求的提高,抗拔桩基础先于基坑开挖施工逐渐成为发展趋势。伴随着基坑开挖产生的大面积卸荷,抗拔桩受力状态和极限承载力将产生重大变化。本文将颗粒流数值模拟与极限平衡理论分析相结合,剖析开挖卸荷条件下,抗拔桩破坏机理和极限承载力特征。分析表明：抗拔桩周边土体的破坏特征为倒锥台形,存在线性破坏面和破坏角;开挖卸荷对桩周土的破坏角影响不大,但桩长对破坏角的影响显著;根据机理研究结果,建立理论分析模型,推导出较实用的抗拔桩承载力及其损失比理论公式,并针对前人的模拟试验结果对理论公式进行了检验。     

An alternative approach to determine critical angle of contrast reversal and surface roughness of oil slicks under sunglint

The critical angle is the angle at which the contrast of oil slicks reverse their contrasts against the surrounding oil-free seawater under sunglint. Accurate determination of the critical angle can help estimate surface roughness and refractive index of the oil slicks. Although it’s difficult to determine a certain critical angle, the potential critical angle range help to improve the estimation accuracy. In this study, the angle between the viewing direction and the direction of mirror reflection is used as an indicator for quantifying the critical angle and could be calculated from the solar/viewing geometry from observations of the Moderate Resolution Imaging Spectroradiometer (MODIS). The natural seep oil slicks in the Gulf of Mexico were first delineated using a customized segmentation approach to remove noise and apply a morphological filter. On the basis of the histograms of the brightness values of the delineated oil slicks, the potential range of the critical angle was determined, and then an optimal critical angle between oil slicks and seawater was then determined from statistical and regression analyses in this range. This critical angle corresponds to the best fitting between the modeled and observed surface roughness of seep oil slicks and seawater.     

一种多角度观测系统几何精度的评估方法

多角度观测系统用于地面遥感实验中测量地物的双向反射特性。配合高光谱分辨率遥感卫星影像,可以反演得到地物的三维结构信息。多角度观测系统的几何精度直接影响到观测地物的范围及采集数据的准确性。对其进行评估,不仅为实验人员选择实验设备提供精度参数,而且有助于实验人员确定观测范围。以激光脚点最小包络圆半径作为评估参数,提供了一种评估多角度观测系统几何精度的方法;对自主研发的多角度观测系统的几何精度进行了计算与评估。对不同光谱仪视场下的理论观测范围与考虑系统几何精度后的实际观测范围进行了计算,并给出了参考。     

Slope stability analysis by means of finite element limit analysis and finite element strength reduction techniques. Part II: Back analyses of a case history

This paper deals with a back analysis of a slope failure. The case history investigated is located in an alpine environment in central Europe and is characterized by a very steep original terrain, indicating in situ soil with high strength. To study the factor of safety, two different approaches applying the so-called φ′/c′ reduction are used, namely finite element limit analysis and strength reduction finite element analysis. Comparison of a strength reduction technique with rigorous finite element limit analysis confirms that the factors of safety (FoS) obtained are very similar for associated plasticity, an intrinsic assumption of limit analysis. For non-associated plasticity, a modified version of the so-called Davis approach has been applied because it has been shown that the original formulation proposed by Davis works well when the FoS is defined in terms of loads but is not appropriate when the FoS is defined in terms of soil strength. The results show that, with the modified Davis parameter, both strength reduction finite element analyses and finite element limit analyses provide very similar factors of safety. The key advantage of limit analysis, however, is that the value of the FoS can be bracketed from above and below with upper and lower bound calculations.